The new sensor provides for the direct measurement of the pH of geothermal brines, water in nuclear reactors, and other high temperature solutions. The heart of the unit is an oxygen ion conducting ceramic that serves as the membrane in a structure somewhat akin to that of the conventional glass electrode. Yttria stabilized zirconias with both aqueous internal junctions and dry, metal‐metal oxide internal junctions were employed in the present work. Test units were run continuously without failure for periods as long as 13 days at temperatures of 285°C and at pressures of 1200 psi. Under these conditions the response over the pH range 3–8 (at 285°) was linear with a slope up to 97% of theoretical. Unlike sensors involving electron transfer couples (e.g., the hydrogen electrode, the palladium hydride electrode, metal‐metal oxide couples) the membrane is insensitive to changes in the redox environment. In turn, the relatively inert membrane exerts no influence on the environment; this behavior contrasts markedly with that of the hydrogen electrode and many other candidates for use at high temperatures. Although primary emphasis was placed on high temperature performance, the ceramic membranes were also shown to function as pH sensors over the temperature range from 25° to 285°C. The conductivities of the yttria stabilized membranes, as measured in sensors, were found to be comparable in magnitude and activation energy with literature values for related materials measured at elevated temperatures in connection with their application in high temperature fuel cells and oxygen sensors.